Occasionally we see an oil leak from the crankcase gasket that seals the two halves of the crankcase together. Or a pinhole develops in the actual casting of the crankcase and oil will seep out of it. Other areas that leak are protrusions through the crankcase such as the governor arm shaft. Head gaskets can be another source of an oil leak out of the crankcase. An oil seal at the crankshaft is another oil leak source.
Note: Be extra cautious during the testing as you are working near a lawn mower blade. You can shutdown the engine prior to checking for where the engine is leaking for safety.
There are some dyes that can be added to the oil to find a hard to find oil leak. You put the dye in the crankcase and run the engine to get it warmed up. Then a black light is used to identify where the oil leak is coming from.
If you don’t have access to this type of testing paraphernalia, thoroughly clean the engine crankcases and then run the engine until it is warm. Look over the areas that you suspect the leak is coming from and look for the slightest weeping of oil. Wipe the area down and continue to monitor it. Some leaks are difficult to find, but you will eventually find them.